1. Field of the Invention
The present invention relates to an image processing apparatus, a printing apparatus, and an image processing method. More particularly, the present invention relates to quantizing, by a dither method or the like, image data that has been subjected to correction such as head shading for suppressing density unevenness caused by variations in print characteristics of print elements.
2. Description of the Related Art
The head shading (abbreviated as “HS”) technique disclosed in Japanese Patent Laid-Open No. H10-13674 (1998) has been known as one example of the correction for suppressing an uneven density of the aforementioned type. This HS technique is adapted to perform correcting based on information on ink ejection characteristics (i.e., print characteristics) of each of nozzles serving as printing elements. For example, in response to information that amount of ink to be ejected by a certain nozzle is more than normal amount, image data is corrected so as to decrease a gradation value indicated by the image data corresponding to the nozzle. In contrast, in response to information that amount of ink to be ejected by a certain nozzle is less than the normal amount, image data is corrected so as to increase a gradation value indicated by the image data corresponding to the nozzle. In this manner, the number of ink dots to be eventually printed is increased or decreased, so that the density of a print image formed based on the image data can become substantially even in each of the nozzles.
However, after the correction by the HS technique or the like, the corrected image data is quantized, and therefore, the number of dots to be eventually printed may be different according to print characteristics such as ejection amount, thereby a pattern of dot arrangement, that is, a spacial frequency on the dot arrangement is different between print elements. As a consequence, although density unevenness in the print elements may be reduced, a difference in the spacial frequency between the print elements may newly cause density unevenness (hereinafter also referred to as frequency unevenness).
In the case where, for example, ink ejection amount by a nozzle A as a print element is a normal value whereas ink ejection amount by a nozzle B adjacent to the nozzle A is larger than the normal value, a dot formed at a position to be printed by the nozzle B may be larger than that formed by the nozzle A. Here, when the data is corrected by HS processing in such a print head, the number of dots to be printed at the position where the nozzle B prints is smaller than that to be printed by the nozzle A during a printing operation based on print data obtained by quantizing the corrected data. Consequently, the number of dots is different between an area having a predetermined area where the dots are printed by the nozzle A and a area having the same area as that where the dots are printed by the nozzle B. That is to say, the same quantization causes different patterns of dot arrangement. This difference in pattern of dot arrangement leads to a difference in spacial frequency of dots, and thus, the difference is visually recognized as frequency unevenness. More specifically, even if the density unevenness caused by each of the nozzles can be overcome by the HS processing, the spacial frequency of the dot to be printed is unfavorably different among the nozzles, thus raising another problem of frequency unevenness.